1. override
void dispose()

Frees all the resources used by this performance. After this is called, this object is no longer usable.

Source 

@override
void dispose() {
  assert(_currentTrain != null);
  _currentTrain.removeStatusListener(_statusChangeHandler);
  _currentTrain.removeListener(_valueChangeHandler);
  _currentTrain = null;
  _nextTrain?.removeListener(_valueChangeHandler);
  _nextTrain = null;
  super.dispose();
}